Degradation mechanisms of axial compressive performance in defective unbonded flexible pipes

As a critical conduit connecting offshore oil and gas fields to marine platforms, the unbonded flexible pipe plays a vital role in offshore production. The tensile armor, as one of the primary load-bearing components of the flexible pipe, is crucial to the structural integrity of the pipeline system, and its failure poses a significant threat. Therefore, investigating the influence mechanisms of various defects on the mechanical properties of the tensile armor and analyzing its mechanical behavior are of great theoretical and engineering significance. A numerical simulation approach was employed to study the axial compression performance of the tensile armor in unbonded flexible pipes with defects. A five-layer finite element model of the unbonded flexible pipe under axial compression was established to examine the effects of defects in the non-metallic layer, interlayer friction coefficient, and wires fracture on the axial compression stiffness and critical buckling load. The results indicate that a decrease in the friction coefficient due to increased annular water content significantly reduces the critical buckling load, while defects in the non-metallic layer and fracture of the wires substantially decrease both axial compression stiffness and critical buckling load. These findings emphasize the necessity of monitoring annular water content and ensuring the structural integrity of each layer in engineering applications. By constructing a critical buckling load degradation model and a nonlinear axial stiffness degradation model considering defects in both non-metallic and metallic layers, as well as a degradation model for the critical buckling load and stiffness that accounts for the differences between internal and external fracture of the metal layer, this study reveals the scale-separation characteristics of interlayer mechanical transmission in unbonded flexible pipes. A key innovation of this work is the development of a dual-scale coupled axial compression degradation model. By introducing a novel degradation factor ($\beta (\theta ,L)$) linking non-metallic defect severity to metallic layer buckling sensitivity and a nonlinear weighted degradation term ($\psi (n_{out},n_{\text{int}})$) capturing the accelerated stiffness loss due to wire fracture interactions, this model explicitly quantifies the synergistic effects between non-metallic and metallic defects, which have largely been treated in isolation previously. This provides a more comprehensive and physically grounded framework for evaluating the axial compression performance of unbonded flexible pipes with complex, co-existing defects.